On August 8th I experienced the same problem. In order to get it to work, the following change was made to the account configuration: “Send / Authentication method” was changed from “Automatic” to “PLAIN”. However, I do not fully understand the security implications of this change, or why it worked - it was kind of a random grasping at straws. Today I also experienced claws-mail hanging and grabbing all the CPU - never happened before - had to kill and restart it.